The present invention relates to an optical recording/reproducing apparatus for optically recording information on a recording medium and reproducing the recorded information.
In conventional optical recording/reproducing apparatuses which record and reproduce information by using laser beams, tracking control is achieved by forming two or more beam spots on a recording medium and leading reflected light beams from the beam spots to a detector, whereby the difference in quantity between the reflected light beams is detected. FIG. 1 shows an optical system of one such conventional optical recording/reproducing apparatus. Referring to FIG. 1, there are shown a laser source 1, a collimator lens 2, a diffraction grating 3, a polarized beam splitter 4, a quarter wavelength plate 5, an objective lens 6, a converging lens 7, and a detector 8 divided in three. A light beam emitted from the laser source 1 is converted into a parallel light beam by the collimator lens 2, and then projected on the diffraction grating 3. The light beam incident on the diffraction grating 3 is split into a main beam 9A traveling along the optical axis of the optical system, and at least two auxiliary beams 9B and 9C directed at a certain angle to the optical axis, which are individually projected on the polarized beam splitter 4. The main beam 9A and the auxiliary beams 9B and 9C incident on the beam splitter 4 are projected with P-polarization, and transmitted through the beam splitter 4 to be led to the quarter wavelength plate 5. Thereafter, the main beam 9A and the auxiliary beams 9B and 9C are projected on the objective lens 6, and converged thereby on an information track 11 of a disk 10 as a recording medium.
The main beam 9A and the auxiliary beams 9B and 9C are reflected by the disk surface, and projected again on the beam splitter 4 via the objective lens 6 and the quarter wavelength plate 5. Since the main beam 9A and the auxiliary beams 9B and 9C are subject to S-polarization as they reciprocate once through the quarter wavelength plate 5, they are reflected by the beam splitter 4 to be led to the converging lens 7. Then, the beams 9A, 9B and 9C are converged on light receiving surfaces 8A, 8B and 8C of the detector 8 by the converging lens 7. Record information on the disk 10 is chiefly obtained from the main beam 9A converged on the light receiving surface 8A, while tracking information is obtained from the auxiliary beams 9B and 9C converged on the light receiving surfaces 8B and 8C.
FIG. 2 is a plan view showing spots of the main and auxiliary beams converged on the disk. In FIG. 2, numeral 21 designates the spot of the main beam 9A, and numerals 22 and 23 denote those of the auxiliary beams 9B and 9C, respectively. If the laser beams converged on the disk 10 are free from any tracking errors, the main beam 9A is on the center line of pits 24, and the auxiliary beams 9B and 9C are on the edges of their corresponding pits 24. Thus, the quantities of reflected light from the spots 22 and 23 are equal. If the laser beams are subject to tracking errors, then the main beam 9A is off the center line of the pits 24, and the auxiliary beams 9B and 9C are off the edges of the pits 24. Thus, the quantities of reflected light are unequal. Accordingly, the tracking information can be obtained by detecting the difference between the quantities of reflected light derived from the auxiliary beams 9B and 9C by means of the light receiving surfaces 8B and 8C of the detector 8.
Thus, in order to obtain the tracking information, the differently angled auxiliary beams 9B and 9C must be independently converged on the light-receiving surfaces 8B and 8C of the detector 8. To attain this, it is necessary that the distance between the converging points of the auxiliary beams 9B and 9C be long enough to match the size of the detector 8.
In the prior art optical recording/reproducing apparatus, however, it is necessary to increase the focal distance of the converging lens 7 if the relative angle between the auxiliary beams 9B and 9C is minute. In consequence, the optical paths are extended to prevent the miniaturization of the optical system. Hereupon, if the slit width of the diffraction grating 3 and the wavelength of the laser source 1 are D and .lambda., respectively, the relative angle .theta. between the auxiliary beams 9B and 9C are given by EQU .theta.=sin.sup.-1 (.lambda./D). (1)
Also, if the distance between the light-receiving surfaces 8B and 8C of the detector 8 is d (constant), and if the focal distance of the converging lens 7 is f, then we obtain the following relation. EQU d=f.multidot..theta.. (2)
Therefore, if the relative angle .theta. between the auxiliary beams 9B and 9C is minute, it is evident from eq. (2) that the focal distance f of the converging lens 7 needs to be increased in order that the beams may be received by the light-receiving surfaces 8B and 8C. This requirement constitutes a serious hindrance to the miniaturization of the optical system.